Contact angle measurements of liquid droplets on substrate surfaces commonly are used to measure wettability of liquids on the substrate surface and to evaluate adhesion. The contact angle is defined as the angle between the substrate support surface and the tangent line at the point of contact of the liquid droplet with the substrate. The value of the contact angle of the liquid droplet will depend upon the surface energy of the substrate and the surface tension of the liquid. If perfect or complete wetting takes place between the liquid and the substrate surface by reason of high surface energy, the droplet will spread out over the substrate and the contact angle will approach 0, whereas if wetting is only partial, the resulting contact angle will lie in the range of 0.degree. to 180.degree.. Devices are known for determining the contact angle of the droplet, both by direct measurement of the angle and by indirect calculation based upon measurements of the height, width, and/or radius of the droplet. Most common procedures involve projecting a silhouette image of the deposited droplet onto a screen and determining the contact angle by direct or indirect measurements taken from the silhouette.
Measurements of contact angles commonly are used in industry, both in research as well as in routine testing of surface properties of materials or articles of manufacture. For example, without specific surface treatment, many plastic materials, such as polypropylene and polyethelene, have insufficient wettability for effectively retaining printing inks applied thereto or for bonding decorations or other materials applied to the surface. Contact angle measuring procedures routinely are used in quality control to ensure that proper surface treatment is effected on the material for the required printing, decorating or bonding. Similar procedures are used in numerous other manufacturing or scientific applications. Accurate and repeatable contact angle measurements heretofore have required utilization of flat, horizontal substrate surfaces. In many instances, however, the surfaces of articles of manufacture to be tested are convex, cylindrical, or otherwise curved. In such cases, the liquid droplets of the test liquid tend to slide along the convex or cylindrical surface preventing accurate contact angle measurement.